1. Field of the Invention
This invention relates to a method and an arrangement for sensing the position of a linear electromagnetic actuator that operates according to the principle of a voice coil.
2. Description of the Related Art
It is known that one can measure the position of an electromagnetic actuator, in which the moving part comprises an iron core arranged so as to be influenced by the magnetic field generated by a stationary coil, by analyzing the inductance of the coil. One example of this method is shown in U.S. Pat. No. 5,172,298. An unavoidable disadvantage of this method is that the relative variation of inductance is low, which causes the absolute accuracy to be low as well. This is clearly a disadvantage if high precision is desired. In U.S. Pat. No. 5,172,298 it is also suggested that the coil may be divided into a driving coil and a measuring coil. This is an additional disadvantage, in that this leads to increased complexity.
In order to quickly position, for example, a hard disk pick-up, U.S. Pat. No. 4,937,510, discloses how one may use analog electronics to analyze the electromotive force (emf) that is induced by movement of the coil and therefrom to measure the coil""s velocity. The absolute position is in this case controlled according to complicated principles and the measurement of velocity is intended to be used only by a velocity regulator that has a large bandwidth. The same type of velocity feedback has also been used to regulate the velocity of the coil in a loudspeaker; however, this often involves including a second, dedicated measuring coil in conjunction with the driving coil. Neither of these two methods is able to produce a value that indicates the absolute position of the actuator.
International Patent Application PCT/SE98/01564 (U.S. Pat. No. 6,246,563, issued 12 Jun. 2001) discloses how yet another actuator principle may be used to determine position by measuring variations in inductance, which are derived from the mutual inductance created by the transformer included in the disclosed type of actuator. The principle relies, however, on a complex structure with respect to both the driving of the actuator and to analysis of the position of the actuator, which in certain applications is undesirable.
The object of the invention is to solve the above-mentioned problems in order to achieve a rapid and accurate measurement of the position of the moving coil in a linear actuator based on the principle of a voice coil. This object is achieved by driving and measuring the position of an actuator, which has a gap that is magnetized by a permanent magnet, and in which gap a voice coil is arranged to move between two end positions, where the amount of core material that is surrounded by the voice coil varies as a function of position. The voice coil is connected to a controllable current source, so that the average current of the voice coil can be controlled. This in turn controls the actuator force. An alternating current component is then also included in the control current, by means of which the phase shift of the circuit can be measured. This phase shift then provides a measure of the position of the voice coil relative to the core material. One advantage of the invention is that it eliminates the need for end point breaker switches.
The invention makes possible the use of the voice coil principle in new applications. Characteristic of the principle of a voice coil is that it forms a quick, bidirectional, highly dynamic force source. When this principle is complemented with a simple and exact method for measuring position, new areas of application are created.
The position information that the invention generates may be used in any number of different contexts. One example is that actual position information can be used in any system that regulates the position of the actuatorxe2x80x94regulators require some information about instantaneous position and this invention provides such information. For example, the invention may be used to generate position-feedback information in a position-regulated system. Yet another example are diagnostic functions for checking and analyzing the operation of the actuator itself.
The voice coil is driven by a current source that can in part control the average current through the voice coil and in part create an AC component. The average current provides for control of the force that the voice coil is to develop and the AC component provides an opportunity to analyze the inductance in the voice coil. The inductance of the voice coil can be caused to vary greatly as a function of position by allowing part of the voice coil to extend outside of the inner core when the voice coil is in its outermost position.
In the preferred embodiment of the invention, the current source comprises one or more switching elements that apply voltage to the voice coil in such a way that the instantaneous current value through the voice coil 3 oscillates between two controllable limiting values. The frequency of oscillation then becomes a measure of the position of the voice coil. A present position value of the voice coil is sensed and is coupled to a feedback position regulator that controls the average current through the voice coil.
In one optimized embodiment of the invention, the temperature in the voice coil is measured, from which a compensation factor is derived and used to compensate the measurement error that is caused by temperature changes in the voice coil.